1. Field of the Invention
The present invention generally relates to a method for semiconductor chip packaging, and more specifically to a method for wafer level package of sensor chip.
2. The Prior Arts
When the integrated circuit (IC) is designed and fabricated, the IC must be packaged. The purposes of packaging is to connect the IC with the other electrical component as to accomplish (1) power transmission, (2) electrical signal transmission, (3) provision of heat dissipation, and (4) carrying and protecting IC.
As the arrival of customized electronic products, single chip products, and portable of audiovisual products, the modern demands of thin, light, short, small, new speedy, inexpensive, and environmental friendly products has driven the packaging technology going through a rapid change. At present, the chip scale package (CSP) is widely adopted.
Conventional IC packaging is to dice the wafer, and the die are placed on conductive lines and glued with silver glue. Then, the contact points on the die are connected by thin gold wire to inner pins of the conductor frame to connect to outside through the conductor frame. Next, the epoxy process is used to seal the die to prevent humidity. Finally, the outer pins of the conductor frame are electroplated with the alloy of Zn and Pb so that the IC can be easily attached to the printed circuit board (PCB). After the seal printing, cutting the pins, formation and frame removal, a complete IC component is completed. Therefore, the conventional IC is packaged at the die level. On the other hand, wafer-level package (WLP) is different from the conventional packaging process. In WLP, the entire wafer is packaged, and the packaged wafer is then diced. The size of the die using WLP is about the same size of the unpackaged die.
The sensor chips of the opto-electronic image sensors, such as charge coupled device (CCD), complementary metal-oxide semiconductor (CMOS) emphasize on receiving EM radiation signals. In general, the surface of the chip includes an active area for receiving EM signals, and an electrical pin, such as a pad, surrounding the active area. When the EM signal reaches the active area, the chip will transform the optical signal of the EM wave into electrical signal, and transmitted through the pad.
The aforementioned sensor chips are usually packaged with a wire bonding technology for electrical connection, and then sealed inside a glass with a supporting structure for protection and light penetration. As this type of packaging process requires a highly clean environment (Class 100), the costs of the facility and operation are both high. In addition, as the volume of the packaging structure is sizeable, the size of the packaged sensor chip cannot be effectively reduced. U.S. Pat. No. 6,396,043 disclosed a flip chip package method using glass as a substrate and light penetration layer to reduce the thickness of the packaged structure and overcome the limitation of the light penetration direction in the flip chip package. However, this method has two drawbacks. One is the area of the packaged chip is still large, and the other is the chip need to be packaged individually.
U.S. Pat. No. 6,528,857 disclosed a package structure with a bump of the chip size. The package structure includes an image sensor chip, whose upper surface includes an active area with a pad surrounding the active area, a window supported by a window support on the sensing area, a step-up ring (multi-layer laminated structure) formed on the upper surface of the image sensor chip and located between sensing area and the pad, and a conductive line (external line) formed on the step-up ring to connect to the pad on the upper surface of the image sensor chip through wire bonding. The signal can go through the pad, the wire bonding, and the external line to reach the solder ball. Other than wire bonding, the patent also disclosed a flip chip packaging method by using the bump to connect the inner line beneath the step-up ring and the pad of the chip so that the signal can go through the pad, the bump, the inner line, the hole, the external line to reach the solder ball. Finally the signal is transmitted to the PCB by the surface mount technology (SMT). With this structure, the size of the packaged image sensor chip is about the original size of the image sensor chip before packaging. The patent pointed out that the packaging method is suitable for batch processing, and applicable to a package structure for an array. After ball attachment to the wafer, the wafer is diced. The drawbacks of this method include the thickness of the final package, the complexity of the process and the bump is prone to damage.
U.S. Pat. No. 6,646,289 disclosed a glass layer WLP structure similar to a sandwich. The front surface of the wafer is pressed on an insulation cover plate (glass plate), and the back surface of the wafer is ground. After the thinning of the wafer, the wafer goes through a conventional etching process to form individual separate dice. Then, the glue is applied to glue the second insulation cover layer (glass plate) to the back surface of the wafer so that the wafer is packaged between two insulation layers. Next, the special cutting tool is used to cut a v-shaped trench on the second insulation layer between two neighboring dice. The depth of the trench is just enough to expose the edge of the pad on the wafer. A layer of metal conductive layer is formed in the v-shaped trench so that the T-contact is extended to the second insulation layer. After the redistribution and the solder ball planting, the wafer is diced to complete the package structure. The packaging method disclosed in U.S. Pat. No. 6,646,289 is a typical WLP method. Its drawbacks include high cost (two glass layers and special cutting tool), difficulty of cutting precision, high error rate, pollution during cutting, and the bump electrode prone to breakage.